Naturally occurring steroid that acts as a supramolecular zipper could have potential as a new anticancer drug.

Orde Munro and co-workers at the University of KwaZulu-Natal, South Africa, have isolated a previously unknown steroid from a Southern African grassland lily, Ornithogalum tenuifolium. The x-ray structure of the compound revealed the steroid monomers stacking to form one-dimensional chains that interlock with neighbouring polymers. The overall structure of the natural product almost perfectly mimics a man-made zip. 

The structure is the first example of a zip composed of steroid molecules. It is formed from small organic building blocks rather than the usual coupled polymer chains such as those found in DNA. The molecules must interlock in two directions, along the length of the zip and across it, which, according to Munro, is 'a truly extraordinary feat'. 

The group used theoretical techniques to model the steroid and successfully reproduced its conformation and extended structure. They found that the zip formation was due to a serendipitous combination of hydrogen-bonding groups held in a unique conformation.

A close isomer of the steroid has been found to be toxic to cells, although its different shape means that it does not 'zip'. Since a molecule's shape can have a profound effect on its biological activity, Munro thinks that the new compound may have enhanced cellular toxicity. He believes this may make the compound of interest to medicinal chemists looking for new anticancer drugs.

Celia A Clarke